This invention is related generally to a system and a method for chemical decontamination of radioactive material, and more particularly to a system and a method for chemically dissolving oxide film on a surface of a contaminated component or the base material of the component.
In a facility handling nuclear radiation, oxide film containing radioactive nuclides is adhered or generated on the internal surface of the constructional parts in contact with fluid containing radioactive material as the operation is continued. When the operational experience time becomes longer, the radiation level around the constructional parts such as piping and components becomes higher, the dosage the personnel would receive during periodic inspection or during demolishing in decommissioning of the facility would be increased. Practical chemical decontamination technique, by which the oxide film is chemically dissolved and removed has been developed to reduce dosage of personnel.
Various chemical decontamination methods have been proposed. For example, a method is known which has a step of oxidizing and dissolving the chromium oxide in the oxide film with oxidizer agent and a step of reducing and dissolving the iron oxide which is a main component of the oxide film by reduction agent.
Japanese Patent Publication (Tokkou) Hei-3-10919 discloses a chemical decontamination method where dicalboxylic acid (oxalic acid) aqueous solution is used as a reducer. According to this method, permanganic acid and oxalic acid are used. Permanganic acid has a strong oxidation effect with low concentration, and oxalic acid can be decomposed into carbon dioxide and water. Therefore, the amount of secondary waste material generation is reduced compared to the conventional chemical decontamination method. This method has been actually used in a decontamination work of a nuclear power facility.
Japanese Patent Application Publication (Tokkai) 2000-81498 discloses a chemical decontamination method where ozone aqueous solution is used as an oxidizer and oxalic acid aqueous solution is used as a reducer. Ozone is decomposed into oxygen, and oxalic acid is decomposed into carbon dioxide and water. Therefore, this method is noted as a decontamination technique which can reduce secondary waste material.
Japanese Patent Application Publication (Tokkai) Hei-9-113690 discloses a method for decontaminating stainless steel waste material in organic acid (oxalic acid or formic acid) aqueous solution. According to this method, a stainless steel component is set in contact with a metal component which has a lower potential than oxidation-reduction potential of stainless steel, and the base material of stainless steel is dissolved and decontaminated. Since a single organic acid aqueous solution process is used, the decontamination process is simple. In addition, since the base metal is dissolved, this method is effective as a method for decontaminating waste metal to a general industrial waste level of radioactivity.
Japanese International Patent Application Publication (Tokuhyou) Hei-9-510784 (International Patent Application Publication WO 95/26555) discloses treatment of oxalic acid aqueous solution as a treatment of decontamination waste liquid. According to this reference, Fe3+ in the oxalic acid aqueous solution forms anions as a complex with oxalic acid. Fe3+ is reduced into Fe2+ by irradiation of ray (hν), as shown in Equation (1) shown below:[Fe(C2O4)3]2−+hν→FeII(C2O4)2+2CO2  (1)
Then, Fe2+ in the oxalic acid aqueous solution can be separated by cation resins. Oxalic acid is decomposed by the oxidation effect of hydroxy radical or OH(radical), which is generated as a result of a reaction of hydrogen peroxide (H2O2) and Fe2+, and carbon dioxide and water are generated as shown in Equations (2) and (3) shown below:H2O2+Fe2+→Fe3++OH−+OH(radical)  (2)H2C2O4+2OH(radical)→2CO2+2H2O  (3)
The techniques disclosed in the references cited above can be used as decontamination techniques for reducing dosage of personnel working for periodic inspection of nuclear facilities such as nuclear power plants. However, ultraviolet ray devices are required to reduce Fe3+ into Fe2+ when oxalic acid is used as a reducer. As the structure to be decontaminated becomes larger, the amount of the decontamination liquid increases, and the required ultraviolet ray device becomes larger, which results in enhanced cost for the device construction. In addition, required time period for dissolving oxalic acid becomes longer which results in longer decontamination work time period.
In the technique disclosed in Japanese Patent Application Publication Hei-9-113690, formic acid is utilized as a decontamination agent. However, formic acid cannot be used in decontamination if the component to be decontaminated has to be in safe, because formic acid electro-chemically dissolves the base metal. Furthermore, simple treatment with only formic acid cannot dissolve and remove oxide film and iron oxide which have been generated on the surface of the components, and sufficient decontamination performance cannot be obtained.
Japanese Patent Application Publication (Tokkai) Hei-2-222597 and Japanese International Patent Application Publication (Tokuhyou) 2002-513163 (International Patent Application Publication WO 99/56286) disclose chemical decontamination techniques for radioactive metal waste. Japanese Patent Application Publication Hei-2-222597 discloses a method where the component to be decontaminated is temporally electrolyzed and reduced in sulfuric acid aqueous solution, and the potential is lowered to corrosion region of stainless steel so that the base metal would be dissolved and decontaminated.
Japanese International Patent Application Publication 2002-513163 cited above discloses a method of decontamination, where trivalent irons are reduced into bivalent irons by ultraviolet ray, and oxidation-reduction potential of organic acid aqueous solution is lowered to corrosion region of stainless steel so that the base metal would be dissolved and decontaminated. This reference also discloses a method for removing iron ions in organic acid aqueous solution by cation exchange resins. Since trivalent irons are in form of complexes with organic acid as complex anions, they cannot be removed by cation exchange resins. Therefore, trivalent irons are reduced into bivalent irons by irradiation of ultraviolet ray. Bivalent irons can be easily removed by cation exchange resins since bivalent iron oxalate complex would be less stable.
According to the technique disclosed in Japanese Patent Application Publication Hei-2-222597 cited above, oxidation-reduction potential is enhanced when concentrations of iron ions and chromium ions dissolved in the decontamination liquid increase. Therefore, dissolving reaction of stainless steel ceases, and the decontamination performance would deteriorate. Since sulfuric acid is used as a decontamination agent, the decontamination waste liquid generated in the decontamination process cannot be accepted in the existing waste liquid process system of nuclear facility without modification. A dedicated neutralization treatment device and an aggregation/settling tank are required. The aggregation/settling tank is to be used for separating deposition, which is separated out as hydroxide, and clear supernatant liquid, which would result in higher cost for construction of the decontamination system. Furthermore, large amount of secondary waste material is generated in the neutralization process, and cost for disposing the waste material increases.
According to the technique disclosed in Japanese International Patent Application Publication 2002-513163 cited above, the decontamination device itself in contact with the decontamination liquid would be corroded, since the potential is lowered by concentration control of the bivalent and trivalent irons in organic acid decontamination liquid. Especially, oxalic acid has larger corrosion rate compared to other organic acids. Therefore, the decontamination device made from stainless steel may have a failure due to corrosion. In addition, the metal removed by the ion exchange resins includes metal which has eluted from the decontamination device, so that another problem may be generated in increase of spent ion exchange resins.
The present inventors have obtained new information by actually decontaminating components contaminated with radioactivity, using the technology disclosed in Japanese Patent Application Publication Hei-9-113690 cited above. The newly obtained information includes:
(1) In a case of using organic acid as decontamination liquid, if only oxalic acid is used, decontamination performance is high because it reduces and dissolves iron oxide. However, it takes long time to decompose the oxalic acid. If only formic acid is used, it takes shorter time to decompose the formic acid compared with the oxalic acid. However, the decontamination performance is not high because formic acid would not dissolve iron oxide.
(2) Similarly to the technology disclosed in Japanese Patent Application Publication Hei-2-222597 cited above, in a case of temporary potential control, oxidation-reduction potential of the decontamination liquid is enhanced, as the concentrations of iron ions and chromium ions dissolved in the decontamination liquid increase. Therefore, dissolving reaction of stainless steel ceases, and decontamination performance deteriorates.
(3) When oxide film including chromium oxide film is generated or adhered on the surface of the component, decontamination performance can be enhanced by oxidizing-dissolving the chromium with oxidizer agent.
The entire contents of the all references cited above are incorporated herein by reference.